This invention relates to a reproduction apparatus which drives a disk type recording medium to rotate at a fixed linear velocity to perform data reproduction, and a rotation control apparatus and a rotation control method applied to a reproduction apparatus of the type mentioned.
A system wherein a disk such as a CD (compact disk) or the like is employed as a recording medium has been popularized. In a system of the type mentioned, recording data for which EFM modulation (Eight to Fourteen Modulation), which is a kind of run length limited codes, has been performed are recorded onto a disk. Further, for an operation of rotating a disk, a CLV (Constant Linear Velocity) system is adopted.
In the related art CLV rotational servo, for example, an EFM signal read out from a disk is inputted to a phase locked loop circuit (hereinafter referred to simply as PLL circuit) to extract a clock, and the extracted clock is compared with a reference clock obtained by a crystal element to obtain rotational error information. Then, the rotational error information is fed back to a spindle motor for rotating a disk so that a rotational condition of a constant linear velocity is obtained.
In order to allow such a CLV servo circuit as described above to function, the PLL circuit must be locked in a condition wherein a clock is extracted accurately. To this end, a construction for performing rough servo control for pulling, upon starting up of the spindle motor, an EFM signal extracted first into a capture range of the PLL circuit is required. In short, in a disk reproduction apparatus, for example, upon starting of rotation of the spindle, rotational servo control is first performed to some degree by a rough servo circuit, and then at a point of time when the PLL circuit is locked, the CLV servo operation is changed over from the rough servo circuit to an ordinary CLV pull-in servo circuit.
A construction of a CLV servo system for a disk reproduction apparatus is shown in FIG. 1.
Referring to FIG. 1, the CLV servo system includes a rough servo circuit 100 and a CLV velocity detection circuit 110.
In the rough servo circuit 100, an EFM signal reproduced from a disk is first inputted to a pit length measurement circuit 101. The EFM signal is run length limited codes which are defined such that a maximum reversal interval of a code train thereof is 11T and a minimum reversal interval is 3T, and the pit length measurement circuit 101 measures the pit length between edges of the inputted EFM signal with reference to a reference measurement clock produced by a crystal (XTAL) element and supplies information of a measured value to a maximum value holding circuit 102. The maximum value holding circuit 102 selectively holds a maximum value from the measurement information of the pit length inputted from the pit length measurement circuit 101 and outputs the maximum values to a minimum value holding circuit 103 in the following stage. The minimum value holding circuit 103 selectively holds a minimum value from the maximum values inputted thereto from the maximum value holding circuit 102 and outputs the minimum value. Thus held value in the minimum value holding circuit 103 exhibits a minimum pit length selected from maximum pit lengths obtained by the maximum value holding circuit 102. In other words, even if the EFM signal exhibits reversal intervals longer than 11T due to a read error arising from, for example, a damage to a disk or the like, they are cancelled, and information of a maximum pit length almost close to 11T is obtained.
While information of a pit length close to 11T which is the longest reversal interval is obtained within a certain range by the minimum value holding circuit 103 in this manner, an 11T detection circuit 104 compares the pit length (reversal interval value) held by the minimum value holding circuit 103 with the pit length of 11T which serves as a reference to output an error signal of three values. In short, the 11T detection circuit 104 outputs comparison signals of three different values among three cases including a case wherein the held value by the minimum value holding circuit 103 and the pit length of 11T which serves as a reference are equal to each other, another case wherein the pit length of 11T which serves as a reference is larger and a further case wherein the pit length of 11T which serves as a reference is smaller. An error signal obtained in this manner is supplied as a pull-in servo signal CLV-1 to a spindle motor not shown in FIG. 1 so that rough servo control for a CLV is performed.
The CLV velocity detection circuit 110 includes a sync pattern detection circuit 111, to which, as seen in FIG. 1, an EFM signal and a signal PLCK (of, for example, 4.3218 MHz) which corresponds to a clock outputted from a PLL circuit (not shown in FIG. 1) for clock extraction are inputted.
At the top of one frame (588 bits) of the EFM signal, a sync pattern of 24 bits is encoded. The sync pattern is formed from fixed patterns of 11T, 11T and 2T from the top of the frame. In the sync pattern detection circuit 111, the sync pattern is detected by counting the inputted EFM signal per pit (in other words, counting is performed for each 588 bits) using the signal PLCK as a reference clock.
A detection output of the sync pattern detection circuit 111 is supplied to an interpolation protection circuit 112. The interpolation protection circuit 112 executes processing of interpolation of the sync pattern, window protection and so forth if the sync pattern is not detected at an original position or the sync pattern is detected at a position at which the sync pattern should not be originally present, for example, due to a dropout of the reproduction signal, an influence of jitters or the like.
Information of the sync pattern outputted from the interpolation protection circuit 112 is branched and inputted to a frame sync production circuit 113 and a velocity counter 114. The frame sync production circuit 113 produces a frame sync signal based on the detection signal of the inputted frame sync, and the frame sync signal is utilized for required signal processing and so forth.
Meanwhile, in the velocity counter 114, the frame sync at a timing synchronized with the signal PLCK is counted with a predetermined frequency generated by a crystal element so that velocity error information may be obtained. The velocity error information is outputted as a velocity detection signal CLV-2. The velocity detection signal CLV-2 is supplied to a driver for the spindle motor not shown in FIG. 1 so that CLV control in a condition wherein the sync pattern is detected (that is, in a condition wherein the PLL circuit is locked) can be performed. It is to be noted that, though not shown here, for CLV control, for example, also a phase error signal obtained by comparing the clock produced by the PLL circuit with the predetermined frequency signal of the crystal element in phase is used together with the velocity detection signal CLV-2.
In the CLV servo system having such a construction as described above, for example, upon starting of rotation of the spindle motor, the system of the rough servo circuit 100 is utilized to perform rough servo control to control the speed of rotation of the spindle motor until the PLL circuit is pulled into its capture range as described hereinabove. Then, in a condition wherein the PLL circuit is locked, changing over from the rough servo circuit system to the system of the CLV velocity detection circuit 110 is performed so as to control the velocity of rotation of the disk to a constant linear velocity.
In the construction of such a CLV servo system as shown in FIG. 1, for example, upon starting of the spindle motor described above, when the CLV servo is lost by vibrations applied from the outside or the like or when a signal drops for a long period of time, changing over to the system of the rough servo circuit 100 is performed so that rough servo control is started again. However, since the pull-in servo signal CLV-1 of the rough servo circuit 100 can assume only three values as described above, the rough servo circuit 100 can perform servo control only in a narrow bandwidth of, for example, 1 Hz or less. Consequently, a comparatively long time is required to restore the condition wherein the PLL circuit is locked again.
In recent years, also with regard to CD players and so forth, portable products have spread widely. However, if a disturbance originating from rolling along the direction of rotation of the disk occurs during a reproduction operation of such a portable CD player or the like, then the relative variation of the speed of rotation of the disk relative to an optical pickup is rendered significantly by such disturbance. Consequently, in such a condition as just described, restoration particularly of a reproduction operation is very difficult.
Further, in the construction of such a CLV servo system as shown in FIG. 1, since two CLV servo circuit systems including the system which includes the CLV velocity detection circuit 110 for ordinary servo control and the system of the rough servo circuit 100 for rough servo control are required naturally, the circuit scale becomes large as much.